Asex organ, also known as areproductive organ, is a part of anorganism that is involved insexual reproduction. Sex organs constitute the primarysex characteristics of an organism. Sex organs are responsible for producing and transportinggametes, as well as facilitatingfertilization and supporting the development andbirth of offspring. Sex organs are found in many species of animals and plants, with their features varying depending on the species.
Sex organs are typically differentiated intomale andfemale types.
In animals (including humans), the male sex organs include thetesticles,epididymides, andpenis; the female sex organs include theclitoris,ovaries,oviducts, andvagina. The testicle in the male and the ovary in the female are called theprimary sex organs.[1] All other sex-related organs are known assecondary sex organs. The outer parts are known as thegenitals orexternal genitalia, visible at birth in both sexes,[1] while the inner parts are referred to asinternal genitalia, which in both sexes, are always hidden.[2]
In plants, male reproductive structures includestamens in flowering plants, which producepollen.[3] Female reproductive structures, such aspistils in flowering plants, produceovules and receive pollen for fertilization.[4]Mosses,ferns, and some similar plants havegametangia for reproductive organs, which are part of thegametophyte.[5] Theflowers offlowering plants produce pollen andegg cells, but the sex organs themselves are inside the gametophytes within the pollen and the ovule.[6]Coniferous plants likewise produce their sexually reproductive structures within the gametophytes contained within thecones and pollen. The cones and pollen are not themselves sexual organs.
Theprimary sex organs are thegonads, a pair of internal sex organs, which diverge intotesticles following male development or intoovaries following female development.[8] As primary sex organs, gonads generate reproductivegametes containing inheritableDNA. They also produce most of the primary hormones that affect sexual development, and regulate other sexual organs and sexually differentiated behaviors.
Secondary sex organs are the rest of the reproductive system, whether internal or external. TheLatin termgenitalia, sometimes anglicized asgenitals, is used to describe the externally visible sex organs.
In generalzoology, given the great variety in organs, physiologies, and behaviors involved incopulation, male genitalia are more strictly defined as "all male structures that are inserted in the female or that hold her near hergonopore during sperm transfer"; female genitalia are defined as "those parts of the female reproductive tract that make direct contact with male genitalia or male products (sperm,spermatophores) during or immediately after copulation".[9][page needed]
It is hard to find a common origin forgonads. However, gonads most likely evolved independently several times.[10] At first,testes andovaries evolved due tonatural selection.[11]
A consensus has emerged thatsexual selection represents a primary factor for genital evolution.[12] Male genitalia show traits ofdivergent evolution that are driven by sexual selection.[13]
The development of the internal and external reproductive organs is determined by hormones produced by certain fetal gonads (ovaries or testicles) and the cells' response to them. The initial appearance of thefetal genitalia looks female-like: a pair ofurogenital folds with a small protuberance in the middle, and theurethra behind the protuberance. If the fetus has testes and the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings grow, wrap around the penis, and fuse in the midline to form thepenile raphe.[17][18] Each organ/body part in one sex has ahomologous counterpart.
The process of sexual differentiation includes the development ofsecondary sexual characteristics, such as patterns of pubic and facial hair and female breasts that emerge at puberty.
Because of the strongsexual selection affecting the structure and function of genitalia, they form an organ system that evolves rapidly.[19][20][21] A great variety of genital form and function may therefore be found among animals.
Other animals
In many othervertebrates, a single posterior orifice (thecloaca) serves as the only opening for the reproductive, digestive, and urinary tracts (if present) in both sexes. Allamphibians, birds,reptiles,[22] some fish, and a few mammals (monotremes,tenrecs,golden moles, andmarsupial moles) have this orifice, from which they excrete both urine and feces in addition to serving reproductive functions.[23] Excretory systems with analogous purpose in certain invertebrates are also sometimes referred to as cloacae.
Penile and clitoral structures are present in some birds and many reptiles.
The organs concerned withinsect mating and the deposition of eggs are known collectively as the external genitalia, although they may be largely internal; their components are very diverse in form.
Planaria are flat worms widely used in biological research. There are sexual and asexual planaria. Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Each planarian transports its excretion to the other planarian, giving and receiving sperm.
In flowering plants, theflowers contain the sex organs.[26]
Sexual reproduction inflowering plants involves the union of the male and female germ cells, sperm and egg cells respectively. Pollen is produced instamens and is carried to thepistil orcarpel, which has the ovule at its base wherefertilization can take place. Within each pollen grain is a male gametophyte, which consists of only three cells. In most flowering plants, the female gametophyte within the ovule consists of only seven cells. Thus there are no sex organs as such.
The sex organs infungi are known asgametangia. In some fungi, the organs are indistinguishable from each other but, in other cases, male and female sex organs are clearly different.[27]
Similar gametangia that are similar are known as isogametangia. While male and female gametangia are known as heterogametangia, which occurs in the majority of fungi.[28]
